Recover and recycle rhodium from spent partial oxidation catalysts

ABSTRACT

A method for the recovery of rhodium from spent supported catalysts. In one embodiment, a method for recovering rhodium from a host material includes roasting the host material in air at a temperature sufficient to convert at least a portion of rhodium to Rh 2 O 3 , leaching the host material in a solution with a leaching constituent which is reactive with Rh 2 O 3  to form a first intermediate species, reacting the first intermediate species in a solution with an acidifying constituent or complexing agent to form a second intermediate species, and purifying the second intermediate species. Preferably, the roasting temperature is approximately from 600° C. to 800° C. for 0.5 to 10 hours. In some embodiments, the host material is ground to particles in the range of 0.1 to 10 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

TECHNICAL FIELD OF THE INVENTION

This invention relates to spent catalyst compositions for partialoxidation processes. More particularly this invention relates to amethod for recovering and recycling rhodium in spent supported partialoxidation catalysts.

BACKGROUND OF THE INVENTION

The separation and purification of rhodium (Rh) from other preciousmetals is one of the most difficult and pressing areas in precious metalrefining. This situation arises mainly because of the complex solutionchemistry in chloride-containing aqueous solutions. The complexes formedby rhodium in these types of solutions are such that modern recoveryprocesses such as solvent extractions (SX), which have been implementedfor the recovery of other platinum group metals (PGMs), cannot be easilyapplied to the recovery of rhodium. Thus far, no industrially acceptablesolvent extraction system has been developed for rhodium.

Rhodium is often used in combination with other PGMs in catalysts. Inthe life of a catalyst, the catalyst may lose some or all of itsactivity. A catalyst may deactivate through the accumulation of a layerof carbon deposits, or coke. Coke accumulation typically occursthroughout the catalyst pore systems and physically blocks access toactive sites. Further, metal agglomeration may occur, which can severelyreduce catalyst activity. Still further, poisons (e.g., lead, arsenic,sulfur) may permanently deactivate the catalyst. In many cases,deactivated catalysts are regenerated so that they recover at least partof their initial activity.

Cycles of deactivation and regeneration may occur for many years. Thecatalyst may be regenerated in situ or removed for ex situ regeneration.In one strategy, a fixed bed or slurry bed reactor unit and aregenerator unit are paired in tandem, for simultaneous operation. Afterthe catalyst is regenerated ex situ it is commonly loaded back to thesame or another unit. This procedure has the advantage of reducing downtime of the reactor. Alternatively, a catalyst may continuouslyrecirculate between a reactor and a regenerator. Cost savings over freshcatalyst vary widely, but using regenerated catalyst can save 50-80% ofthe new catalyst cost.

A catalyst that has been through cycles of use and regeneration may,with time, lose the ability to be regenerated to an adequate level ofactivity, becoming a spent catalyst. This loss of regenerability may bedue to incompleteness of the regeneration. For example, in an oxidativeregeneration of a coked catalyst, sulfur in the coke is typically notremoved to as low a level as coke is removed during regeneration.Further, sulfates associated with alumina supports are typically notremoved, nor are metal poisons. Permanent loss of acceptable activitymay also occur through sintering or other structural changes.

Often a spent catalyst is discarded. However, a spent catalyst, ifdiscarded, represents a loss of precious material, such as rhodium.Further, use of landfills for such disposal is problematic. For example,available landfills have decreased in number by 75% in the past 20years, a trend that is expected to continue. Further, environmentalliability can reach unacceptable levels if the landfill releases toxinsto the environment. Still further, the environmental protection agency(EPA) “Land Ban” imposes restrictions on disposal.

Thus it is desirable to have a method for reclamation of catalystmaterials. Reclamation is the process of recovering and recycling amaterial. For a PGM-containing catalyst, reclamation is particularlydesirable for economic reasons. For example, a single drum of spentcatalyst may contain thousands of dollars worth of valuable metals, suchas rhodium, platinum, palladium, iridium, ruthenium, and osmium.

In particular, it is desirable to have a method for reclamation ofrhodium from a spent catalyst. Rhodium is a relatively scarce materialand is accordingly rather expensive. The costs of the entire catalyticprocess could be reduced appreciably by recovery of the rhodium fromspent catalysts and subsequent recycling of the metal.

Because multiple PGMs are often used together, it is important to devisetechniques to separate them and to purify and recover each of the metalsseparately. Originally, PGMs were separated after dissolution inoxidizing chlorine leach liquors by the application of a series ofprecipitation-dissolution steps adopted from analytical chemistrymethods. This was the most common route until the middle nineteenseventies. Since then, the major refining companies have considerablymodified their processes by implementing the more efficient separationtechnique of solvent extraction, and to a lesser degree, ion exchange.

In virtually all precious metal recovery systems, rhodium is the lastmetal recovered through a complicated precipitation technique ratherthan through the more modern and efficient technique of solventextraction. The precipitation scheme-dissolution scheme for the recoveryof rhodium is not considered satisfactory by most PGM refiners becauseof its numerous drawbacks. It is a lengthy process, sometimes taking aslong as 4 to 6 months for the final recovery of pure rhodium metal andtherefore, there is a high value of metal that is locked up in theprocessing plant. The technique is also quite tedious, as theprecipitation must be carried out a number of times in order to ensurethat the final product is of acceptable purity and this makes theoverall process labor intensive and costly.

In the precipitation-purification method, the first step involves theformation of the nitrite complex [Rh(NO₂)₆]³⁻ from RhCl₆ ³⁻. Becausethis complex is extremely stable to hydrolysis, the impurerhodium-containing solution can be subjected to neutralization with NaOHin order that some of the impurities be precipitated through hydrolysis.After a filtration stage, the rhodium in solution is precipitated withammonium and sodium (from the NaOH) as Na(NH₄)₂[Rh(NO₂)₆], which is apartially selective precipitation step over the other PGMs that may alsobe present in the rhodium solution. For this precipitation, however, itis important that a high concentration of ammonia be used in order tosuppress the solubility of this rhodium complex to achieve almostcomplete rhodium precipitation. After another filtration stage, theprecipitate is redissolved in HCl and, depending on the purity of thesolution, the process recommences with the nitrating step.

It is this cycle of precipitation-dissolution stages that renders thisprocess inefficient and labor-intensive. Once the ammonia-nitriterhodium complex is of acceptable purity, the final dissolution in HCl isfollowed by the precipitation of rhodium with ammonia to give(NH₄)₃[RhCl₆]. It is not only important that the concentration ofammonia be high to suppress the solubility of the rhodium compound, butalso that the chloride concentration be high, since it is thehexachloro-complex that is precipitated and therefore thehexachloro-complex must be available in solution. The last step involvesthe reduction of rhodium to the metallic state either directly from thissolution with formic acid or by calcining the complex in the presence ofH₂ gas at about 1000° C.

As described above, rhodium metal is of high value, and with rapidlyincreasing demand for catalysts that utilize rhodium, the need todevelop more efficient recovery processes such as solvent extraction forrhodium is becoming more urgent. Particularly, a method for recoveringrhodium from solid spent catalysts is needed. The difficulty indeveloping such systems, however, lies in the chemical complexity ofrhodium in Cl-containing aqueous solutions.

The main oxidation state of rhodium is +III, although +I and others areknown to exist, though to a much lesser extent. The anionic complexes ofrhodium are more labile than those of other PGMs, whereas the cationicand neutral complexes are quite inert.

Rhodium (III) readily forms octahedral complexes, as do most d6configurations, with anions, halides, and oxygen-containing ligands. Interms of solvent extraction, highly charged RhCl₆ ³⁻ ions areparticularly difficult to extract due to steric effects because it isdifficult to pack three organic molecules around a single anion.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for therecovery of rhodium from spent supported catalysts, and in particularfrom spent supported partial oxidation catalysts. The methods of thepresent invention overcome some of the drawbacks of existing rhodiumrecovery techniques. The new rhodium recovery methods require fewersteps and are economically feasible for use under commercial-scaleconditions.

To separate Rh from a solid phase (i.e., on ceramic support), an acid orother leach solution is used. This initial separation step is done bywashing Rh from a column packed with spent catalyst, as if spentcatalysts were ion-exchange resins. Only metals originally supported onthe catalysts are removed to solution, and the residual solids (largelyceramic support) are discarded after the leaching process. Themetal-containing solution is then acidified and purified further viaion-exchange reactions. Acidification with HCl transforms Rh³⁺ intoRhCl₆ ³⁻, an anionic form, so Rh can be separated from other co-existingcationic metals, such as Sm³⁺ on an anionic ion-exchange column. Othermethods to make anionic form of Rh is to use nitrite (NO₂—), orpseudohalide (CN— or SCN—) acids or salts.

In accordance with a preferred embodiment of the present invention, amethod for recovering rhodium from a solid host material includesroasting the host material in air at a temperature sufficient to convertat least a portion of rhodium to Rh₂O₃, leaching the host material in asolution with a leaching constituent that is reactive with Rh₂O₃ to forma first intermediate species, reacting the first intermediate species ina solution with an acidifying constituent or complexing agent to form asecond intermediate species, and purifying the second intermediatespecies. Preferably, the roasting temperature is approximately from 600°C. to 800° C. for 0.5 to 10 hours. In some embodiments, the hostmaterial is ground to particles in the range of 0.1 to 10 mm. Theleaching constituents may be chosen from the group including HCl, HNO₃,H₂SO₄, HClO₄, HCN, HSCN, and complex ligands (e.g. ethylenediaminetetraacetic acid (EDTA)). As used herein, a complex is defined as astable association of a metal with an organic ligand having one or morenitrogen, oxygen or sulfur atoms with a lone pair of electrons (i.e.aliphatic amines). Rh can be removed by ligands from the supports byforming a dissolved Rh complex. The acidifying constituents andcomplexing agents may be chosen from the group including HCl, HNO₂, andmixtures with their respective ammonia or sodium salts. In a preferredembodiment, the second intermediate species is separated by an anionicion-exchange column.

In accordance with another preferred embodiment of the presentinvention, a method for recovering rhodium from a spent catalystincludes removing at least a portion of the rhodium from the catalystvia an acid or ligand solution and extracting the rhodium from the acidor ligand solution.

In accordance with yet another preferred embodiment of the presentinvention, a method for reclaiming rhodium from a spent catalystincludes removing at least a portion of the rhodium from the catalystvia an acid or ligand solution, isolating the rhodium from the acid orligand solution, and incorporating the isolated rhodium in a freshcatalyst.

These and other embodiments, features and advantages of the presentinvention will become apparent with reference to the followingdescription.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The selection of a catalyst or catalyst system requires many technicaland economic considerations. The process of selecting a catalyst can bebroken down into components. Key catalyst properties include highactivity, high selectivity, high recycle capability and filterability.Catalyst performance is determined mainly by the active metalcomponents. For example, a metal might be chosen based both on itsability to complete the desired reaction and its inability to completean unwanted reaction. Additionally, a catalyst may also include asupport and may take any desired shape, including monolithic, spherical,etc.

Rhodium is often used in partial oxidation catalyst systems because itgives extremely high activity in the dehydrogenation of hydrocarbons.Specifically, rhodium is capable of dehydrogenation at moderatetemperatures and pressures.

The present invention includes a method for recovery rhodium from spentcatalyst using the following steps:

roasting the host material in air at a temperature sufficient to convertat least a portion of rhodium to Rh₂O₃;

leaching the host material in a solution with a leaching constituentthat is reactive with Rh₂O₃ to form a first intermediate species;

reacting the first intermediate species in a solution with an acidifyingconstituent or complexing agent to form a second intermediate species;and

purifying the second intermediate species.

In one embodiment, the host material is a spent partial oxidationcatalyst. The host material may be porous, such as a foam or monolith,or may be nonporous, such as a solid sphere. When the host material isporous, it is preferable to grind the host material to particles in therange of 0.1 to 10 mm to expose more rhodium. It is not necessary togrind nonporous structures.

During the roasting, the rhodium component is oxidized to Rh₂O₃. In thiscase, the process takes place in the presence of excess air attemperatures of approximately 600° C. to 800° C. for 0.5 to 10 hours.

In a preferred embodiment of the present invention, following roastingand grinding if necessary, the host material is packed into a leachingcolumn as slurry with deionized (DI) water. Once the column issufficiently packed, a leaching constituent is used to leach out therhodium from the host material at a predetermined rate. The leachingconstituent is preferably chosen from the group including HCl, HNO₃,H₂SO₄, HClO₄, HCN, HSCN, and complex ligands (e.g. ethylenediaminetetraacetic acid (EDTA)). In a preferred embodiment, the leachingconstituent is dilute HCl, ranging from 0.5M to 5M HCl. Preferably, theleach rate is approximately 0.1-1% bed volume per minute to achievesufficient separation and reduced total volume. This solution containingthe first intermediate species (i.e. RhCl₃) may then be concentrated,preferably by evaporation on a hot plate at a temperature below theboiling point of the solution.

Once first intermediate species is concentrated, it is reacted with anacidifying constituent or complexing agent. The acidifyingconstituent/complexing agent is preferably chosen from the groupincluding HCl, HNO₂, HCN, HSCN, and mixtures thereof with theirrespective ammonia or sodium salts. In a preferred embodiment, theacidifying constituent is concentrated HCl, of at least 6M HCl.Acidifying or complexing the first intermediate species preferably formsan anionic, second intermediate species (i.e. [RhCl₆]³⁻).

The anionic, second intermediate species is then purified or separatedfrom other cationic metal ions (e.g. Cu²⁺, Yb³⁺, Mn²⁺, Ni²⁺, Zn²⁺,Sm³⁺). Preferably, the second intermediate species is separated by ananionic ion exchange column with NH₄Cl, NH₄NO₂, or NaNO₂ as elutingagents for other cationic metal ions. The form of eluting agents (i.e.,either chloride or nitrite salt) should be the same as the reactingagents in the previous step. After removing the cationic metal ions, theadsorbed Rh complexes can be recovered by eluting with a base,preferably NH₄OH. In a preferred embodiment, the eluting rate isapproximately 0.1-1% bed volume per minute.

Ion exchange is a reversible chemical reaction wherein an ion, an atomor molecule that has lost or gained an electron and thus acquired anelectrical charge, is exchanged from solution for a similarly chargedion attached to an immobile solid particle. The solid ion exchangeparticles are preferably naturally occurring inorganic zeolites orsynthetically produced organic resins. The synthetic organic resins arethe predominant ion exchange material used today because theircharacteristics can be tailored to specific applications.

An organic ion exchange resin is composed of high molecular weightpolyelectrolytes that can exchange their mobile ions for ions of similarcharge from the surrounding medium. Each resin has a distinct number ofmobile ions that set the maximum quantity of exchanges per unit ofresin.

Ion exchange resins are classified as cation exchangers, which havepositively charged mobile ions available for exchange, and anionexchangers, whose exchangeable ions are negatively charged. Both cationand anion resins are produced from the same basic organic polymers. Theydiffer in the ionizable group attached to the hydrocarbon network. It isthis functional group that determines the chemical behavior of theresin. Resins can be broadly classified as strong or weak acid cationexchangers or strong or weak base anion exchangers.

Ion exchange processing can be accomplished by either a batch method ora column method. In the first, the resin and solution are mixed in abatch tank, the exchange is allowed to come to equilibrium, then theresin is separated from solution. The degree to which the exchange takesplace is limited by the preference the resin exhibits for the ion insolution. Consequently, the use of resins exchange capacity will belimited unless the selectivity for the ion solution is far greater thanfor the exchangeable ion attached to the resin. Because batchregeneration of the resin is chemically inefficient, batch processingion exchange has limited potential for application.

Alternatively, passing a solution through a column containing a bed ofexchange resin is analogous to treating the solution in an infiniteseries of batch tanks. As a result, separations are possible even whenthere is poor selectivity for the ion being removed.

An example of the reactants and products for rhodium separation andpurification according to one preferred embodiment are shown below:

1) Roasting 2Rh + 3O₂ → 2Rh₂O3 2) Leaching Rh₂O₃ + 6HCl (dilute) →2RhCl₃ + 3H₂O 3) Acidification/ RhCl₃ + 3HCl (concentrated) → [RhCl₆³⁻ + 3 H⁺ Complexation 4) Purification* 3R⁺ -[RhCl₆]³⁻ + 3NH₄OH →RhCl₃ + 3NH₄Cl + 3R⁺-OH 5) Concentration RhCl₃ + nH₂O → RhCl₃• nH₂O *R⁺represents the functional group of an ion-exchange resin

Step 1 is preferably carried out in air at 600-800° C. Step 5 ispreferably carried out at 60-100° C. to isolate Rh as RhCl₃·nH₂O. Insome embodiments (i.e. a purification requiring a faster drying time),step 4 may be carried out in a vacuum.

In a preferred embodiment, the product (i.e. RhCl₃·nH₂O) is kept as aRhCl₃ stock solution with predetermined Rh concentrations. This stocksolution is preferably used for partial oxidation catalyst preparation.

Catalyst

A host material, or rhodium-bearing catalyst, according to the preferredembodiments of the present invention may include any suitable supportmaterial. Preferably, the support is a catalyst support. The catalystsupport may be any of a variety of materials on which a catalyticallyactive material may be coated. The catalyst support preferably allowsfor a high degree of metal dispersion. The choice of support is largelydetermined by the nature of the reaction system. The support catalyst ispreferably stable under reaction and regeneration conditions. Further,it preferably does not adversely react with solvent, reactants, orreaction products. Suitable supports include activated carbon, alumina,silica, silica-alumina, carbon black, TiO₂, ZrO₂, CaCO₃, and BaSO₄.Preferably, the catalytically active material is supported on carbon,alumina, zirconia, titania or silica.

It will be understood that alternative choices of support may be madewithout departing from the preferred embodiments of the presentinvention by one of ordinary skill in the art. A support preferablyfavorably influences any of the catalyst activity, selectivity,recycling, refining, material handling reproducibility and the like.Properties of a support include surface area, pore volume, pore sizedistribution, particle size distribution, attrition resistance, acidity,basicity, impurity levels, and the ability to promote metal-supportinteractions. Metal dispersion increases with support surface area.Support porosity influences metal dispersion and distribution, metalsintering resistance, and intraparticle diffusion of reactants, productsand poisons. Smaller support particle size increases catalytic activitybut decreases filterability. The support preferably has desirablemechanical properties, attrition resistance and hardness. For example,an attrition resistant support allows for multiple catalyst recyclingand rapid filtration. Further, support impurities preferably are inert.Alternatively, the support may contain additives that enhance catalystselectivity.

As described above, the recovered rhodium is preferably recycled for usein an active, or fresh, catalyst. The catalysts of the present inventionmay be prepared by any of the methods known to those skilled in the art.By way of illustration and not limitation, such methods includeimpregnating the catalytically active compounds or precursors onto asupport, extruding one or more catalytically active compounds orprecursors together with support material to prepare catalystextrudates, and/or precipitating the catalytically active compounds orprecursors onto a support. Accordingly, the supported catalysts of thepresent invention may be used in the form of powders, particles,pellets, monoliths, honeycombs, packed beds, foams, aerogels, granules,beads, pills, cylinders, trilobes, extrudates, spheres or other roundedshapes, or another manufactured configurations.

The most preferred method of preparation may vary among those skilled inthe art, depending for example on the desired catalyst particle size.Those skilled in the art are able to select the most suitable method fora given set of requirements.

One method of preparing a supported metal catalyst is by incipientwetness impregnation of the support with an Rh stock solution. Anothermethod of preparing a supported metal catalyst is by a melt impregnationtechnique, which involves preparing the supported metal catalyst from amolten metal salt. For higher metal loading, the methods may be repeateduntil desired loading is achieved.

In some embodiments, additional promoters and/or base metals may beused. The prepared catalysts are preferably used in partial oxidationoperating conditions.

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present invention to itsfullest extent. The following embodiments are to be construed asillustrative, and not as constraining the scope of the present inventionin any way whatsoever.

What is claimed is:
 1. A method for recovering rhodium from a hostmaterial containing rhodium and other species, the method comprising:(a) roasting the host material in air at a roasting temperaturesufficient to convert at least a portion of rhodium to Rh₂O₃; (b)leaching the host material in a solution with a leaching constituentthat is reactive with Rh₂O₃ to form a first intermediate species; (c)reacting the first intermediate species in a solution with an acidifyingconstituent or complexing agent to form a second intermediate species;(d) purifying the second intermediate species from the other species;and (e) converting the second intermediate species to a final productand recovering the final product.
 2. The method according to claim 1wherein the roasting temperature is approximately from about 600° C. to800° C.
 3. The method according to claim 2 wherein the step (a) occursfor approximately 0.5 to 10 hours.
 4. The method according to claim 1wherein the host material is essentially nonporous.
 5. The methodaccording to claim 1 wherein the host material is porous.
 6. The methodaccording to claim 5 wherein the host material is ground to particles inthe range of 0.10 to 10 mm.
 7. The method according to claim 1 whereinthe leaching constituent is selected from the group consisting of HCl,HNO₃, H₂SO₄, HClO₄, HCN, HSCN and complex ligands.
 8. The methodaccording to claim 7 wherein the leaching constituent is HCl.
 9. Themethod according to claim 8 wherein the leaching constituent is diluteHCl, ranging from 0.5M to 5M HCl.
 10. The method according to claim 9wherein the leaching constituent is cycled to the host material at aleach rate of approximately 0.1 to 1% bed volume per minute.
 11. Themethod according to claim 8 wherein the first intermediate species isessentially RhCl₃.
 12. The method according to claim 1 wherein theacidifying constituent or complexing agent is selected from the groupconsisting of HCl, HNO₂, and mixtures with their respective ammonia orsodium salts.
 13. The method according to claim 12 wherein theacidifying constituent is HCl.
 14. The method according to claim 13wherein the acidifying constituent is concentrated HCl, of at least 6MHCl.
 15. The method according to claim 13 wherein the secondintermediate species is essentially [RhCl₆]³⁻.
 16. The method accordingto claim 1 wherein the host material is placed in a leaching column forstep (b).
 17. The method according to claim 1 wherein the secondintermediate species is placed in an ion exchange system for step (d).18. The method according to claim 17 wherein the ion exchange system isan ion exchange column.
 19. The method according to claim 1 wherein thesecond intermediate species is separated by an anionic ion exchangecolumn to form a product.
 20. The method according to claim 19 whereinthe product is kept as a rhodium-containing stock solution withpredetermined rhodium concentrations.
 21. The method according to claim19 wherein the product is essentially RhCl₃·nH₂O.
 22. A method forreclaiming rhodium from a spent catalyst, the method comprising: (a)removing at least a portion of the rhodium from the catalyst via an acidsolution; (b) isolating the rhodium from the acid solution; and (c)incorporating the isolated rhodium in a fresh catalyst by loading acatalyst support with the reclaimed rhodium.
 23. The method according toclaim 1 wherein step (e) includes eluting the final product with a base.